Black Mask is not just "a guy". He is the insanely evil and dangerous supervillain who tortures people to insanity and death for crap and giggles, and who made Selina's life a terrifying hell. Some of his victims are Steph herself, Selina's brother-in-law, and Selina's sister (who was force-fed parts of her husband's corpse).

Seriously, Black Mask was so utterly and completely bad, even Batman didn't blame Selina for killing him, he just went all "I don't like it, but it's ok, I understand why you did it."